3.1. Constant Load Tests (Creep)
As expected, the sample elongation in time (creep) was observed under constant load conditions (
).
Figure 3a shows examples of creep strains of the sample
at five stress levels (see
Figure 1), while
Figure 3b presents the results of creep tests in the coordinate system (
). The calculation of creep energy
was performed with the use of relation (3).
In all creep diagrams (
Figure 3a) up to the instant of fracture, three characteristic stages with different rates of elongation of the specimen in time are visible. These stages have been marked on the diagram obtained for the lowest stress (
= 219 MPa): stage I (decreasing strain rate), stage II (constant strain rate), and stage III (increasing strain rate). The stress level
affects both the duration of these stages and the elongation rates in individual stages. Based on
Figure 3a, it can be concluded that the magnitude of the strain at fracture slightly decreases with increasing stress. In the case of creep strain energy dissipated until failure,
(
Figure 3b), the influence of the stress level on its value is even less pronounced; the values of creep strain energy at the five stress levels
are very similar. Small differences in creep strain energy at failure confirm that the assumption of constant creep energy as a creep failure condition is correct [
6,
8,
22,
23].
3.2. Fatigue Tests
Instantaneous values of the loading force and deformation of the sample recorded during the tests were used for the analysis of the hysteresis loop parameters.
Figure 4 shows examples of hysteresis loops at two stress levels:
= 219 MPa and
= 258 MPa (see
Figure 1).
As expected, during fatigue loads under the conditions of controlled stress
, cyclic creep of the material is observed as an increase in the subsequent maximum loop strain
. The evolution of
in a fatigue test is influenced by the stress amplitude (see
Figure 5a). In addition to the increase in the relative elongation
with the growing value of
, the range of loop plastic deformation
also increases (
Figure 5b). Changes in the above-mentioned hysteresis loop parameters versus the load cycle number are shown in
Figure 5a,b.
The shift of the hysteresis loops along the strain axis is a consequence of the asymmetry of strains during the tension–compression half-cycle. While the strain asymmetry was not observed under the conditions of elastic strains, the phenomenon becomes apparent when the yield point is exceeded. When the stress exceeds the yield point, the P91 steel exhibits slightly higher resistance to deformation during compression. Based on the analysis of the loops shown in
Figure 4, it can be concluded that with the increase in stress, the deformation asymmetry also increases, which results in the shift of the hysteresis loop. Strain asymmetry during tension and compression was observed and analyzed in many studies [
5,
6].
The increase in plastic deformation
during tension-compression load cycles allows the conclusion that P91 steel belongs to the class of materials exhibiting cyclic softening. This phenomenon was also observed under the conditions of controlled deformation [
6].
The increase in the relative elongation of the sample,
, as well as the change in plastic strain,
, are the result of the loading conditions (stress control,
). On the other hand, under the conditions of
(strain-controlled tests with total strain amplitude
), cyclic creep does not occur. For this reason, strain-controlled tests are preferable, because the existence of creep during stress-controlled tests hinders the analysis of test results [
19]. Based on the research described in the literature [
5,
6], it can be concluded that changes in loop plastic strain
under the conditions of
are much larger than changes in this parameter under
. The consequence of variations in loop plastic strains is the alteration of the plastic strain energy produced in one load cycle,
. Based on the recorded instantaneous values of the loading force and deformation of the sample, the energies
for subsequent load cycles were calculated and are presented in
Figure 6.
The increase in plastic strains with the growing number of load cycles, observed in
Figure 5b, causes the energy of the hysteresis loop
to change as well. This is because the energy
takes into account the interactions between the parameters of the hysteresis loop, such as
,
. The amount of energy changes is influenced by the level of the alternating load (
). As the stress amplitude increases, also the changes in loop energy
are larger (
). As expected, the loop energy
, increases with increasing stress amplitude.
The results of fatigue tests at the temperature of 600 °C under the conditions of
are shown in
Figure 7 in the form of a classic fatigue plot in the log
-
semi-logarithmic coordinate system. To illustrate the influence of temperature on fatigue life, the figure additionally includes the results of fatigue tests of P91 steel samples at room temperature (
= 20 °C). Fatigue plots have been approximated by straight lines with the equations shown in the figure. The analysis of the slope of the fatigue diagrams shows the higher sensitivity of P91 steel to changes in stress at elevated temperature (
). Therefore, slight changes in stress at elevated temperatures may lead to significant changes in durability. For this reason, accurate fatigue life prediction is a key problem for the design of objects operating at elevated temperatures. One should also be aware that the low-cycle fatigue plots in terms of stress are arbitrary due to the presence of significant plastic strains, see
Figure 4.
Based on the results obtained from constant amplitude fatigue tests,
Figure 8 shows the energy accumulation waveforms at five stress levels.
The values of energy cumulated in the sample until failure,
, obtained at individual stress levels, were approximated by a straight line described by Equation (2). The graphs of energy accumulation at individual stress levels are characterized by a clear progression. This is due to the increase in loop energy
, observed in
Figure 6. Significant variations in the energy
raise doubts whether the results obtained with the use of Equation (1) are correct. To illustrate the problem, the table placed in
Figure 8 summarizes the critical energy values
at individual stress levels, estimated in two different ways: (1) experimentally, as the sum of the energies of all hysteresis loops, and (2) calculated as the product of the loop energy from half the fatigue life by the number of cycles to failure (Equation (1)). The results presented in the table clearly show that the energy calculated according to Equation (1) is always lower than the energy obtained from tests, with no clear influence of the stress amplitude level on the differences between the experiment and calculations. The differences in the cumulative energy
from calculations and tests confirm that the Formula (1) may be too simplified and does not account for constant changes in cyclic properties of P91 steel, which belongs to the class of materials that do not exhibit a stabilization period. This problem was addressed in the literature many times, for example in works [
23,
24].
3.3. Alternating Creep-Fatigue Tests (c-f)
Figure 9 shows plastic strain energy
versus the number of a cycle for two levels of cyclic stress amplitude: the lowest (
= 219 MPa) and
= 249 MPa for samples previously subjected to creep during time
= 0.5
. For comparison,
Figure 9 additionally contains the energy functions for a constant-amplitude cyclic load (
) only, performed with the same amplitude levels.
The analysis of the results allows for the conclusion that preceding the cyclic load (f) with a constant load (c), apart from reducing the durability, shifts the plastic strain energy in the first load cycle concerning the load program without permanent loads. When a creep load (in the c-f program) is applied before cyclic loading, the energy of the first hysteresis loop (point 1) corresponds to the hysteresis loop energy in the pure fatigue (f) program, but after a certain number of load cycles was performed (point 1’). It is interesting that only a slight effect of the creep stress level on the initial energy shift was observed. At the same time, the level of significantly influences “the distance” between plastic strain energy evolution curves of (c-f) and (f) loading programs. Moreover, there is a certain level of the hysteresis loop energy for both (c-f) and (f) cases, which separates the zones of lower and higher energy rates (points 2 and 2’). It is noteworthy that this energy is very similar in both runs. The above may suggest that a certain constant loop energy value is necessary for the initiation of a fatigue crack at a given stress amplitude level, and it seems not to depend on the load history.
The test results were next used for the analysis of energy accumulation in both (c-f) and (f) load schemes.
Figure 10 shows the course of energy accumulation for two stress levels, realized under the conditions of a cyclic load only (f), and a cyclic load preceded by a creep load (c-f). Additionally, the critical value of energy according to Equation (2) is presented in the figure.
Energy accumulation under the (c-f) program conditions starts from the value of the energy dissipated in the creep test (
). Based on the results, it can be concluded that the total values of the energy (
) for the samples under the conditions of programmed loads are very close to the boundary diagram for pure fatigue, resulting from Equation (2) (see also
Figure 8).
3.4. Alternating Fatigue-Creep Tests (f-c)
During the implementation of the load variant (f-c), the creep load was preceded by the fatigue load. The loop energy changes for the fatigue part of this test are the same as for the constant-amplitude load (f) (
Figure 6). However, the accumulation of energy in the whole (f-c) test is different. The accumulation of plastic strain energy in the (f) part of the test is then followed until failure by the accumulation of creep energy (see
Figure 11). The whole curves
1 and
5 correspond to the energy plots in
Figure 8. In the points indicated in the figure, the load switches from fatigue to creep, and the cumulative energy paths switch from 1 to 1’, and from 5 to 5’. As a result, the critical values of the total accumulated energy at failure are much lower than in the case of a pure fatigue load test.
In
Figure 11 it can be seen that the critical values of the energy accumulated in the sample during the (f-c) program are definitely below the limit line described by Equation (2). The values of energy cumulated in samples during the implementation of two load variants (c-f and f-c, see
Figure 1) are compared in
Table 1. A pronounced effect of the load sequence on both the critical energy and the total number of cycles is observed.
Based on the above results, it can be concluded that the load sequence has a significant impact on the sample durability. Preceding a fatigue load with a constant load (c-f program) causes a reduction in fatigue life concerning the test in which creep is preceded by fatigue. The load sequence in the program affects also the values of the creep and plastic strain energies dissipated in the sample during the test. In the case of the (c-f) test, the creep strain energy
Wc is several times lower than the relevant energy dissipated in the sample during the (f-c) program. The differences in creep strain energies
Wc between both load programs are related to different creep strain rates observed in
Figure 3. In the case of the (c-f) test, the creep time
τ = 0.5
Tc is related to small creep strain rates of stages I and II. As a result, the
Wc energy is low. On the other hand, in the (f-c) program, the creep load causes a significant increase in the elongation (stages II and III), and as a result, the creep strain energy increases significantly. The opposite situation occurs for the energy
Wf which, in the (c-f) test is twice as large as the plastic strain energy dissipated in the (f-c) program.
3.5. Accumulation of Damage
The paper presents the experimental verification of the linear damage summation model under the conditions of simultaneous creep and fatigue [
7]. The verification was carried out with the use of the durability results, as well as the energy values summarized in
Table 1 and
Figure 8. Total damage,
, was calculated from relation (6).
Table 2 contains the damage values resulting from the fatigue load (
), creep (
), and the total damage
.
A comparative analysis of the total damage levels and its components listed in
Table 2 reveals that, according to the summation hypothesis (6), the fatigue loads have a dominant effect on the durability of the samples during the (c-f) program variant (
), while creep damage was larger for the (f-c) variant (
). To better illustrate the values of total damage
and its fractions
and
, the results are plotted in
Figure 12.
The levels of total damage different from unity confirm the influence of the sequence of the type of load on durability. The damage values also indicate that the linear model of damage summation under conditions of alternating creep and fatigue load results in qualitatively different lifetime predictions, depending on the load sequence. In the case of the (f-c) program, the durability obtained from simplified calculations is lower than that obtained from tests ( > 1), so the lifetime predictions are underestimated. However, under the conditions of the (c-f) load sequence, the durability from calculations is higher than that from the tests ( < 1), so it is overestimated.